Milton L. Lee

Linear retention index system for polycyclic aromatic compounds : Critical evaluation and additional indices

Abstract A linear retention index system for temperature-programmed capillary gas chromatography of polycyclic aromatic compounds using naphthalene, phenanthrene, chrysene, and picene as internal standards has been used in this laboratory for routine analysis since 1979. Conclusions from a data set of 29 routine capillary gas chromatographic analyses of a complex mixture of polycyclic aromatic compounds are used to evaluate, describe, and justify the polycyclic aromatic compound retention index system. The number, width, and linearity of the interpolation intervals are described. The effects on the retention indices of variations in programming rate, column internal diameter, and initial temperature are discussed. The polycyclic aromatic compound retention index system is shown to be a consistent, useful tool for preliminary identification of polycyclic aromatic compounds in complex mixtures. Tables listing standard retention indices for polycyclic aromatic hydrocarbons, polycyclic aromatic sulfur heterocycles, and nitrogen-containing polycyclic aromatic compounds are included.

Hand-portable gas chromatograph-toroidal ion trap mass spectrometer (GC-TMS) for detection of hazardous compounds

A novel gas chromatograph-mass spectrometer (GC-MS) based on a miniature toroidal ion trap mass analyzer (TMS) and a low thermal mass GC is described. The TMS system has an effective mass/charge (m/z) range of 50–442 with mass resolution at full-width half-maximum (FWHM) of 0.55 at m/z 91 and 0.80 at m/z 222. A solid-phase microextraction (SPME) fiber mounted in a simple syringe-style holder is used for sample collection and introduction into a specially designed low thermal mass GC injection port. This portable GC-TMS system weighs <13 kg (28 lb), including batteries and helium carrier gas cartridge, and is totally self-contained within dimensions of 47×36×18 cm (18.5×14×7in.). System start-up takes about 3 min and sample analysis with library matching typically takes about 5 min, including time for column cool-down. Peak power consumption during sample analysis is about 80 W. Battery power and helium supply cartridges allow 50 and 100 consecutive analyses, respectively. Both can be easily replaced. An on-board library of target analytes is used to provide detection and identification of chemical compounds based on their characteristic retention times and mass spectra. The GC-TMS can detect 200 pg of methyl salicylate on-column. n-Butylbenzene and naphthalene can be detected at a concentration of 100 ppt in water from solid-phase microextraction (SPME) analysis of the headspace. The GC-TMS system has been designed to easily make measurements in a variety of complex and harsh environments.

Free radical cross-linking in the preparation of non-extractable stationay phases for capillary gas chromatography

Abstract Free radical cross-linking of methylpolysiloxane stationary phases to form insoluble rubbers recently became of widespread interest in capillary column gas chromatography. The use of various peroxides as free radical generators to form carbon—carbon cross-links in conventional stationary phases has been reported. In this study the applicability of various peroxids and azo compounds fro free radical cross-linking and the effects that these free radical generators have on commercially available stationary phases are described. Characteristics of the stationary phases such as chain length and functional groups and their roles in the cross-linking mechanism are discussed. The properties of benzoyl peroxide, dichlorobenzoyl peroxide, dicumyl peroxide, tert. -butylperoxide, and azo- tert. -butane as free radical generators were evaluated. Stationary-phase polarity change, column activity change and loss of stationary phase were all considered in the overall evaluation of the effectiveness of these free radical generators. Azo- tert. -butane showed minimal effect on phase polarity and column activity while benzoyl peroxide and dicumyl peroxide were the most effective in forming insoluble stationary phases.

Practical aspects of ultrahigh pressure capillary liquid chromatography.

A novel pressure-balanced injection valve was evaluated for use with ultrahigh pressure liquid chromatography (UHPLC) at pressures up to 120 MPa (1,200 bar). Fused-silica capillaries (30-33 cm x 100 microm I.D.) packed with nonporous 1.5 microm isohexylsilane-modified (C6) silica particles were employed to study maximum pressure, injection reproducibility, injection time, and sample amount consumed for an injection. The new valve was more reproducible, convenient, and required much less sample than previously used injection systems. The effect of column diameter on efficiency and sensitivity was studied. The 100 microm I.D. columns demonstrated approximately 40% lower efficiency but 10-fold higher sensitivity than the 29 microm I.D. columns. Columns packed with nonporous C6 particles produced higher efficiencies than columns packed with a 1.5 microm porous octadecylsilane-modified (C18) material.

Identification and geochemical significance of some aromatic components of coal

Abstract The aromatic fraction of a Homestead, Kentucky, coal extract was analyzed by combined gas chromatography-mass spectrometry using glass capillary columns. The aromatic fraction was further analyzed by gas chromatography using sulfur specific flame photometric detection. Seventy-eight compounds were identified, some of which have been traced to their possible biological origins. Sulfur-containing aromatic compounds are thought to originate from reaction of elemental sulfur or pyrite with hydrocarbons.

Capillary supercritical-fluid chromatography with conventional flame detectors

Abstract Coupling of a supercritical-fluid chromatograph to flame-ionization and nitrogen-thermionic detectors is described. A fused-silica capillary restrictor was used to maintain pressure in the analytical column and to expand the effluent into the detector flame jet. Selected applications demonstrate the usefulness of these gas chromatographic detectors in supercritical-fluid chromatography.

Monolithic columns containing sol–gel bonded octadecylsilica for capillary electrochromatography

A new method for preparing monolithic capillary columns is reported. A fused-silica capillary packed with porous octadecylsilica (ODS) particles using a CO2 slurry was partially filled with a siliceous sol formed by hydrolysis and polycondensation of tetramethoxysilane and ethyltrimethoxysilane. After gelling and aging of the siliceous sol at room temperature, the column was dried with supercritical CO2. The scanning electron micrograph of a cross-section of the capillary column revealed that the ODS particles were bonded to each other and to the column inner wall by the sol–gel, forming a monolith. The performance of the monolithic column was evaluated for capillary electrochromatography using small aromatic compounds and polycyclic aromatic hydrocarbons with Tris buffer in aqueous acetonitrile mobile phase. The electroosmotic flow velocity increased with a decrease in Tris buffer concentration from 10 to 5 mM. The electroosmotic flow velocity increased significantly with an increase in Tris buffer pH from 5 to 7.5, and then remained nearly constant with Tris buffer pH between 7.5 to 9.0. The electroosmotic flow velocity increased with an increase in acetonitrile content from 20 to 50%, and then remained nearly constant with acetonitrile content between 50 to 90%. No bubble formation was observed during CEC operation, and all of the test compounds eluted as symmetrical peaks. The logarithms of the retention factors of the aromatic compounds and polycyclic aromatic hydrocarbons decreased linearly with an increase in acetonitrile content. For a 21/29 cm×75 μm I.D. monolithic column containing 9% sol–gel bonded 5 μm ODS, approximately 2.7×104 theoretical plates (plate height 7.8 μm; 1.3×105 theoretical plates per meter) were measured. It was found that the resistance to mass transfer of unretained thiourea was small, which was attributed to partial filling of the small pores in the particulate packing and partial filling of the interstitial space by the porous sol–gel network. The procedure for column fabrication was simple, and three columns were prepared without failure.

Determination of catecholamines and metanephrines in urine by capillary electrophoresis–electrospray ionization–time-of-flight mass spectrometry

A method successfully coupling capillary electrophoretic separation to time-of-flight mass spectrometric (TOFMS) detection for the simultaneous analysis of catecholamines (dopamine, norepinephrine, and epinephrine) and their O-methoxylated metabolites (3-methoxytyramine, normetanephrine, and metanephrine) is described. The inner capillary wall was coated with polyvinyl alcohol in order to obtain baseline resolution of catecholamines and metanephrines and to ensure reproducibility without extensive restorative washing of the capillary. Using electrokinetic injection, detection limits of 0.3 microM for dopamine and norepinephrine, 0.2 microM for 3-methoxytyramine and normetanephrine, and 0.1 microM for epinephrine and metanephrine were achieved with standard solutions. The usefulness of this approach was demonstrated by applying the developed method to the analysis of a spot collection of human urine from a healthy volunteer. The catecholamines and metanephrines were removed from the urine samples and preconcentrated by simultaneous SPE on cation-exchange sorbents. The recoveries of all analytes, with the exception of epinephrine (75%), were over 80%. Catecholamines and metanephrines in the urine samples were quantitated using 3,4-dihydroxybenzylamine as an internal standard. Submicromolar concentrations, consistent with the catecholamine and metanephrine levels reported for normal human urine, were detected.

Direct fluid injection interface for capillary supercritical fluid chromatography-mass spectrometry☆

Abstract A new interface for capillary column supercritical fluid chromatography-mass spectrometry (SFC-MS) is described and initial results are presented. The advantages of SFC include the ability to separate high-molecular-weight, non-volatile and thermally unstable compounds not amenable to gas chromatography. Capillary column SFC-MS has potential advantages aver high-performance liquid chromatography-MS owing to the higher possible chromatographic efficiency, mobile phase volatility and the simplicity of the interface design. The direct fluid injection interface provides for transfer of the total capillary SFC effluent into a chemical ionization source. Initial results are presented to illustrate the separation and analysis of simple mixtures of aromatic hydrocarbons and styrene oligomers using n -pentane as the mobile phase.

Theoretical treatment of resolving power in open tubular column supercritical fluid chromatography

Abstract Theoretical considerations of column pressure drop, column diameter, pressure programming, injector and detector volumes, connector tubing, stationary phase film thickness and detector time constant as applied to open tubular supercritical fluid chromatography are treated. These theoretical considerations show that open tubular supercritical fluid chromatography is a viable technique offering its greatest potential in the pressure—temperature region around the critical point of the mobile phase.